Control system for web heat treating apparatus

ABSTRACT

A temperature sensing means for ascertaining the temperature of a heated moving web includes a sensory means generating an electric signal responsive to the radiation emanating from the web. A control means generating a signal in opposition to the signal of the sensory means provides an error signal equal to the difference between the desired and sensed temperatures of the web. The temperature sensing means includes means for compensating for the emissivity of the web. The temperature sensing means may be incorporated in a temperature control system having an electromechanical means responsive to the error signal for controlling the heat supplied by a web heating apparatus to the web so that the actual temperature of the web equals the desired temperature.

O United States Patent 13,552,645

[72] Inventor Shelton F. Boyd 2,540,966 2/ 1951 Swain 236/ 1 5 Menomonee Falls, Wis. 2,611,541 9/1952 Gray..... 236/15 [21] Appl. No. 782,653 2,752,473 6/1956 Hage 236/15X [22] Filed Dec. 10, 1968 3,163,406 12/1964 Stelling, Jr., et a1. 236/15X [45] pagmed 1971 Primary Examiner-William E. Wayner [73 1 Ass'gnee Attorney-James E. Nilles a corporation of Wisconsin [54] CONTROL SYSTEM FOR-WEB HEAT TREATING APPARATUS 10 Claims, 6 Drawing Figs.

[52] US. 236/15, 73/355, 219/502, 236/84, 250/210, 263/3 [51 Int. Cl. F27b 9/28, GOlk 7/14, GOlk 13/04 [50] Field ofSelrch 73/355,

[56] References Cited UNlTED STATES PATENTS Schmitt ABSTRACT: A temperature sensing means for ascertaining the temperature of a heated moving web includes a sensory means generating an electric signal responsive to the radiation emanating from the web. A control means generating a signal in opposition to the signal of the sensory means provides an error signal equal to the difierence between the desired and sensed temperatures of the web. The temperature sensing means includes means for compensating for the emissivity of the web. The temperature sensing means may be incorporated in a temperature control system having an electromechanical means responsive to the error signal for controlling the heat supplied by a web heating apparatus to the web so that the actual temperature of the web equals the desired temperature.

| l l l I l l l l l l l I l PATENTEDJAN smn 3,552,645

SHEET 1 OF 2 PATENTED JAN 5 I97! sum 2 0i 2 lllllllllllllillIlllllllllllll'tl] 6T Wa CONTROL SYSTEM FOR WEB HEAT TREATING APPARATUS BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates, in general, to control systems and, more particularly, to temperature sensing means for measuring the temperature of a moving web.

2. Description of the Prior Art The present invention may be employed with a web heating oven such as is employed as a web dryer in printing or converting apparatus. In such apparatus, the web is unwound from a storage roll, passed throughthe printing apparatus and down a web heating oven, and then rewound onto a rewind roll. The salient advantage of such web treatingapparatus is the increased processing speed obtainable over equipment'which handles the material in individual sheets, plates, or other forms.

Accurate temperature regulation or control is essential to obtaining the increased processing speed available from web treating apparatus. In the case of a web converting press, the upper limit in the speed at which the converting pressmay be run is often determined by the time interval during which it takes the ink, dye, or paint applied to the web to dry, since until such agents have dried, the web cannot be rewound. Drying of the web becomes extremely critical in cases in which the web is a plastic, such as polyurethane, as there is no absorp tion of the ink by the web to assist in drying the web. By raising the temperature of, or heating, the web, the inkdrying-time is lessened and the. processing speed increased. However, if the temperature of the web is too low, the. web'will' not dry prior to rewinding. On the other hand, if the. temperature of the web is too high, the web may be damaged by the excessive temperature. g

Accurate sensing of the temperature of the moving web is important to web-temperature control. It will be readily .ap preciated, however, that accurate. sensing ofthe-temperature of a rapidly moving web. is difficult. Contact sensors may be employed to sense the temperature of the web by conductionf However, the friction betweenthe sensor and the .web distorts the temperature sensingsthus obtained-Means positioned adjacent the web to sense the temperature of the lamina of air moved by the web have also been employed to measure the web temperature by convection, In general, both conduction and convection temperature sensing means have left much to be desired in terms of both accuracyof the temperature sensing and the speed at which the'devices detect changes in the temperature of the web.

To overcome the shortcomings of conduction and convection temperature sensors, attempts'have been made to utilize radiation sensors which measure thetemperature of the mov ing web by its emitted radiation, thereby avoiding contact with the web or the surrounding air, In the past, radiation sensors have generally been responsive totheinfrared radiation of the web. Because the infrared radiation is buta small portion of r the available radiation spectrum, a-high degree of amplification was required in the temperature sensing apparatus toprovide usable signal levels. The stability, drift, and noise problems encountered in providing such high amplification have caused serious defects in the operation of temperature control systems incorporating such sensors.

Further, the temperature range of such systems is limited'in' minimum temperatures which maybe sensed, as the already small amount of infrared radiation'becomes unusably'small as v the surface to emit radiation compared to an ideal surface. The ideal surface is termed in physics an ideally black surface, or more generally, a black body. The radiation from a less than ideal'surface includes reflected as well as emitted radiation and, in the case of transparent or translucent material, may include transmitted radiation. vlt is generally difficult to predetermine the emissivity factor of a given web material in a form usable for control purposes. It will be appreciated that variations in the emissivity of the web due to changes in the material or other factors may cause variations in the temperature sensing of the radiation sensor, not reflected by actual variations in the true temperature of the web.

SUMMARY or THE PRESENT INVENTION It is, therefore, the'object of the present invention to provide an improved web temperature control'system incorporating a temperaturesensing means fo'r'measuringthe tempera ture of the moving web by means of the emitted radiation of the web.

It is another object of the present invention to provide an improved web temperature control system which utilizes all the radiation emitted by the web so as to provide usable electric signals responsive to web temperature without undue amplification.

Yet another object of the present invention is to provide an improved web temperature control system which includes meansfor compensating for the emissivity of the 'web material, thereby to provide accurate sensings of true web temperature.

Aft'irther object of the present invention is to provide an im proved web temperature control system incorporating the aforesaid temperature sensing means and means for altering heat applied to theweb responsive to the sensed web temperature so'as to bring the actual temperature of the web'into accordance with a desired temperature.

A'still further object of the present invention is to provide an improved web temperature control system which is not subject to drift, calibration errors, noise'instabilities, and other operational shortcomings and which provides stable operation over a wide temperaturerange.

An additional'objectof the present invention is to provide an improved web temperature control system which is simple and inexpensive in construction and operation and which is capable of substantially'trouble free operation for substantial periods of time.

Briefly, the control system of the present invention includes a temperature sensing means for ascertaining the temperature of a heated moving web. The temperature sensing meansutiL izes the radiation which emanates from the web in accordance with both its temperature and an unknown emissivity factor.

The temperature sensing means includes a sensory means positioned adjacent the web and exposed to the radiation emanating therefrom." The sensory means is responsive to the radiation for providing an electric signal corresponding to the amount of the radiation. This sensirig'means may include 'a radiation responsive thermo'pile for rendering the operation of thesensory means stable'and substantially drift free;

Control means'including electric'signal means are connected to the sensory means for providing anelectric signal of a preselected magnitude in electrical opposition to the electric signal of the sensory means. The control means also includes deviation sensing means responsive to the-electric signal's for indicating when the electric signals are in a condition of magnitude equality.

The temperature control system includes means for altering one of the electric signals from the sensory meansor the electric signal means to compensate for the emissivityof the'moving web, so that-the electric signals are in thecondition of magnitude equality when' the temperature of the'web equals the desired temperature. The deviation sensing means, in the form of a null balance, high gain amplifienprovides'an error from the condition of magnitude equality.

The error signal may be-used to drive electromechanical means connected to a controllable heating means in the web heat treating apparatus for controlling the heating means in accordance with the error signal for causing the temperature of the web to equal the desired temperature.

BRIEF DESCRIPTION or THE DRAWING FIG. 1 is a schematic drawing of a web heating oven and the control system of the present invention;

FIG. 2 is a schematic diagram of, one embodiment of the temperature sensing means and control system of the present invention;

FIG. 3'is aschematic diagram of another embodiment of the temperature sensing means; and

FIGS. 4a, b, and c are graphs illustrating the operation of an null type amplifier incorporated in the control system.

DESCRIPTION OF THE PREFERRED EMBODIMENT Web Heating Apparatus Oven A drying oven is of the enclosed tunnel type having an exhaust conduit 6 extending from adjacent the entry end of the tunnel. A series of blast tubes 7- extend inside the tunnel and across its width and act to produce a direct and localized blast of air on the web W as it moves rapidly through the tunnel on idler rolls 8 and in the direction indicated by the arrow in FIG. 1. Web W may comprise plastic, paper paper, cloth, rubber, metal or other material which may beheat treated in the web form. The supply conduit 9 furnishes heated air to one end of each of the tubes from a heating chamber C in which is located burner B. If a more complete description of the structure and operation of the blast tubes or oven is deemed necessary or desirable, reference may be had to U.S. Pat. No. 3,074,179 issued on Jan. 22, 1963, entitled Web Dryer," which shows a typical web heating apparatus.

' Air Supply A fan F is provided for forcing air past the burner B through conduit 9, out of the tubes and over the surface of the web. In the illustration shown, the burner is located on the pressure side of the fan F but of course the burner could be located on the suction side of the fan. The air supplied by the fan is preferably filtered and may consist of fresh air mixed with some of the air which has been exhausted from the oven 0. In other words, the system may be of the recirculating or nonrecirculating type, with the burner located on the suction or pressure side of the circulating fan..

Fuel Valve Means are provided for furnishing a properly proportioned mixture of air and fuel to the burner. This means consists of a conventional mixing or ratio valve mechanism V having an air' inlet opening 10 and a fuel supply conduit 11 for supplying fuel, such as gas, to the valve mechanism. The valve mechanism is located on the inlet side of the blower 12, and the gas and air are intimately mixed and then delivered by blower 12 through its outlet conduit 12a to the burner. These ratio valves are conventional and may be of the type shown in US. Pat. No. 2,286,173, issued July 9, I942, and further description of them is not believed to be necessary except to say that the valve mechanism serves to vary the volume of air as received from a supply source at any pressure, and to vary the volume of the fuel as received from the supply source at any pressure, and to modify the. proportioning so obtained, and to establish such modification for different stages of the feed flow of the proportioned air and fuel; by such facility, the

combustible mixture ultimately resultant from said propor tioning, being of consistency which is dictated by the conditions under which the burner is operated. In short, the fuel mechanism is opened or closed to varythe amount of fuel by a linkage L to thereby vary the heat delivered by the burner.

WEB TEMPERATURE SENSING MEANS AND CONTROL SYSTEM Radiation Sensing Head and Associated Circuitry Radiation sensing head 16 is positioned so as to be exposed to the radiation emitted by web W after it has been heated by the hot air from blast tubes 7. As shown moreclearly in FIG. 2, radiating sensing head 16 may comprise a housing 18 suitable for mounting the head at the exit end of oven 0, within oven 0, or at some other suitable location. The front of housing 18 comprises a piece of clear glass or plastic 20 suitable for the transmission of the entire spectrum of emitted radiation from web W into the interior of housing'l8. The wavelength and frequency of the spectrum "of emitted radiation in any given instance depends on the material and temperatures involved. In the case of a plastic web heated to a temperature of approximately 150, the spectrum includes radiation having wavelengths of from approximately .2 microns up to 20 microns. Mirror 22 is provided at the rear of housing 18 for collecting the aforesaid emitted radiation and for focusing the radiation on thermopile 24 which may comprise a plurality of series connected thermocouples. The resulting rise in the temperature of thermopile 24 due to the impingement of .the focused radiation caused causes the thermopile to generate a voltage proportional to its temperature, which in turn is proportional' to the temperature of web W.

The output voltage of radiation sensing head 16 is provided to conductors 26 and 27. Resistor 29. and resistor bridge 31 containing resistors 33, 35 and 37 andnegative coefficient resistor 39 are used to provide ambienttemperature or cold junction compensation to radiation sensing head 16 to compensate for the temperature existing -in oven 0. Resistor bridge 31 is energized by current supply 43 to alter the magnitude of the output voltage of radiation sensing head 16 to effect the desired compensation.

The thermopile 24 of radiationsensing head 16 provides an output signal of a magnitude sufficientto be usable without amplification, thereby eliminating drift and calibration problems in the sensory means. A radiation head of the type which may be employed in the control system of the present invention is marketed by the Westronics Corp. of Fort Worth,

Texas under the designation 9E7 Ardonox Radiation Unit.

The output of radiation sensing head 16 in conductors 26 and 27 are supplied to electric signal means 30] which provides an electric signal of a preselected magnitude in electrical opposition to the electric signal generated by radiation sensing head 16.

The electric signal means 301- includes resistor bridge 32 located in conductor 26, the input terminals 34 and 36 of which are connected to power supply 38. Bridge terminal 40 is connected to conductor 26 while output terminal 41 is connected to conductor 261.

Terminal 34 of rectifier bridge 32 is connected to the wiper of potentiometer 42 connected in parallel with resistor 44. Resistor 48 and tap 50 are provided in the bridge leg between terminals 34 and 41. The foregoing elements may be used to alter the range of the output signal, if .desired. Resistor 52 is included in the bridge leg between terminals 36 and 41 and resistor 54 is inserted in the other leg terminating at terminal 36. All resistors in rectifier bridge 32 are of a type having a low -temperature coefficient and which are therefore relatively unaffected by the ambient temperature and changes therein.

The operation of electric signal means 301 is therefore rendered stable over the operating range of the temperature sensing means and the control system.

A potentiometer 56 is connected between input terminal 34 and terminal 40. The wiper 56a of potentiometer 56 is connected to terminal 40 and conductor 26.- Adjustment of potentiometer 56 provides a means of inserting the signal of preselected magnitude in electrical opposition to the signal from radiation sensing head 16 and'forthis purpose the potentiometer may comprise a precision wound, ten turn potentiometer. Potentiometer 56 may have'a dial or other means reading directly in degrees of temperature for relating the electric signal generated by the potentiometer to a temperature level.

The power supply 38 for resistor bridge 32 includes a transformer 66 having a primary winding 68 connected to alternating current supply lines 70 and a center tapped secondary winding 72. A pair of diodes 74 are connected to the ends of secondary winding 72 and to positive bus 76. A center tap of secondary winding 72 is connected to negative bus 78. A filter capacitor 80 is connected across busses 76 and 78 and in parallel with transistor 82. Transistor 82 :clips the outputof the power supply 38 at a low level so as to make the output of the.

power supply relatively unaffected by changes in line voltage and the like. A zener diode may be used for the same purpose, if desired. The output of power supply 38 is provided to terminals 34 and 36 and conductors 84 and 86.

The Amplifier signal noise rejection characteristics of such an amplifier.

Electric signal means 301 may be conveniently incorporated in the input of amplifier 30, also as shown in FIG. 2.

: Conductors 26land 27 are connected to a low level, pulse amplitude modulation magnetic preamplifier circuit 302 which provides an amplified errorsignal corresponding to any difference between the electric signals produced by radiation sensing head 16 and electric signal means 301. Null balance magnetic amplifier 90 of circuit 302 includes a saturable magnetic core (not shown) having input or control windings, output or 'lead windings, and bias windings wound thereon. Specifically, a pair of input windings 92 and 94 are connected to conductors v27 and 261, respectively. Input windings 92 and ing 96. Winding 98 provides the necessary operating bias to amplifier 90 from a bias supply comprised of resistor 100 and potentiometer 102. v

A meter 104 is connected across output windings 96 to provide a visual indication of the polarity of the error signal. Positive and negative feedback signals from output winding 96 are provided in magnetic amplifier 90 to control the gain and stability of the amplifier. A positive feedback signal, developed across resistor 106, is provided tocontrol winding 92 in conductor 108. An overall amplifier '30ffeedback signal is provided to control winding 92 in conductor 109, as hereinafter described. A negative feedback signal. developed across resistor 110 is provided to control winding 94 in conductor 112.

The output of magnetic preamplifier 'circuit 90 is provided to operational amplifier circuit 303 in conductor 114. Operational amplifier 116, which may also be of the magnetic amplifier type, includes a saturable magnetic core (not shown) having input windings 118 and 120 wound thereon. The current signal output of operational amplifier 116 is developed in output winding 122. r

A negative feedback signal for operational amplifier 116 is developed in potentiometer 125 1-24 and supplied to input winding 120 in conductor 126. Conductor 126 includes additional resistive and capacitive elements, 127 and 129, which provide and integrating characteristic to amplifier circuit 303 so that the magnitude and polarity of the amplifier circuit output signal is such as to tend to force the input signal to the amplifier circuit to zero. The feedback signal in conductor 109 supplied to the input of preamplifier circuit 302 is also developed by potentiometer 124. Winding 128 provides the necessary operating bias to operate amplifier 116 from a bias supply including resistors 130 and 132' and potentiometers 134 and 136. The amplifier may be energized by winding 138 connected to alternatingcurrent supply lines 140 and 142.

Capacitor 144 between output conductors 146 and 148 removes ripple from the output signal of amplifier 30.

Breakover diode 150 prevents excessive output signals from overdriving subsequentportions of the control system.

Emissivity Compensation Means In almost all materials, the emissivity, that is, the efficiency of the material as a radiator, is less than ideal. In order to avoid false temperature sensing, means must be included-in the control system to correct for this lessened efficiency.

In the embodiment of the invention shown in FIG. 2, the emissivity compensation means comprises a means for physically compensating for web material emissivity so as to eliminate the effects of emissivity'from the operation of the temperature sensing means and control system. For this purpose, a U-shaped trough 151 is provided in the control system through which web W may pass. Trough 151 is positioned opposite radiation head 16, so that glass 20 views only trough 151. Trough 151 may comprise a lower plate 153, which lies below the web and is at least as wide as the web W, an end plate 155, and an upper plate 157 which is narrower than lower plate 153.

The plates forming trough 151 contain heating elements 159, such as resistance wires, embedded in them for heating purposes. Elements 159 may be provided with power from adjustable power supply 161'. The temperature of the plates forming trough 151 is closely controlled by means of thermocouples 163, also embedded in the plates, which regulate the operation of power supply 161. The manner in which trough 151 eliminates the effects of the emissivity of web W is described infra under Operation.

The embodiment of the control system of the present invention shown in FIG. 3 incorporates a means for compensating for the effects of emissivity electrically. Specifically, a potentiometer 165 is provided in shunt withpotentiometer 56 so as to alter the operation of that potentiometer in a manner hereinafter described to compensate for the effects of web emissivity. An additional fixed resistor 167 may also be provided in shunt with potentiometer 156.

Electro-Pneumatic Transducer The output signal of operational amplifier 30 is provided to I an electropneumatic transducer 150 which receives the current signal in conductors 146 and 1 48 and alters the pneumatic output pressure of the transducer in direct proportion to the magnitude of the current input signal. a

For such purposes transducer 150 may include a solenoid 152, the coil of which is connected to conductors 146 and 148. The position of plunger 156 of solenoid 152 is a function of the energization of coil 154. Cavity 158 in the body of transducer 150 contains a diaphragm 160 positioned across the cavity and containing cylindrical valve seat 162. A spacer 164 having a plurality of vents 166 is positioned between plunger 156 and valve seat 162 tocouple the former to the latter. A spring 167 biases valve seat 162 against the force exerted by spacer 164. 7

An exhaust port 168 connects theupper portion of cavity 158 to the surrounding atmosphere. An output port 170 is located in the lower portion of cavity 158 to provide the out put pressure to air line 172.

A valve stem 174 positioned in cavity 158 has a truncated conical portion 176 which closes valve seat 162 when the valve stem is in the upper position and the valve seat is in the lower position. A second conical portion 178 closes valve seat 180 between cavity 158 and chamber 182. Spring 184 positioned in chamber 182 biases valve stem 174 upward. Supply air line 186 supplies inlet air to chamber' 182.

An electropneumatic transducer which may be employed in control system 5 as transducer 150 is made and sold by the Fisher Governer Company, Marshalltown, lowa under the model designation type 546.

Actuator Actuator 188 operates ratio valve mechanism V which con trolsthe heat delivered by burner B through linkage L.

Operation To initiate the operation of the temperature sensing means and the control system, electric signal means 301 is energized from alternating current supply lines 70. Power supply 38 applies a voltage to input terminals 34 and 36 of resistor bridge 32 and wiper 56a of potentiometer 56 provides a portion of this voltage to the terminal 40.

The emissivity compensation means is then operated to compensate the subsequent operation of the control system for the effect of the emissivity of the web material. In the embodiment shown in FIG. 2, the temperature of trough 151 is initially raised to the desired temperature of web W and held at that temperature by means of thermocouples 163 and power supply 161. For example,'the temperature of trough 151 may be established at 150 degrees. Radiation sensing head 16 receives the emitted radiation from trough 151 and provides an output signal to electric signal means 301.

Thewiper 56a of potentiometer 56 is adjusted so that the potentiometer generates an electric signal which exactly opposes the output signal of radiation sensing head 16. Under these conditions, no output signal is supplied to the input of preamplifier circuit 302 due to the condition magnitude equality in the opposing electric signals. This equality or null condition may be observed by meter-104 which senses any deviation between the output signal of the radiation sensing head and the signal generated by wiper 56a of potentiometer 56.

With amplifier 30 in a null condition, the remainder of the control system and web heating oven are operated to provide heat to web W as it passes down oven on idler rolls 8. The web also passes through trough 151 and between thetrough and radiation sensing head 16 so that the radiation now received by the head is a combination of the radiation emitted by trough 151 and the radiation emitted by web W.

Under such conditions, if the temperature of web W is the same as that of trough 151, i.e. 150, the output of the radiation sensing head 16 is not altered and the temperature sensing means continues to provide a signal to amplifier 30 indicating a temperature sensing of 150. As the web W is at the same temperature as trough 151, variations in the emissivity of the web do not cause variations in the output signal of radiation sensing head 16, since any radiation reflected or transmitted by web W comes from trough 151 which is regulated by thermocouples 163 and power supply 161 to the desired temperature of 150 so that radiation sensing head 16 still sees only 150 temperature radiation. Thus, the emissivity of the web material is eliminated as a factor in the temperature sensing means and the control system.

If the temperature of web W is higher or lower than the temperature of trough 151, the radiation received by radiation sensing head 16 is altered as the web passes between trough 151 and radiation sensing head 16. The output signal of radiation sensing head 16 to electric signal means 301 is also altered. The altered output signal of radiation sensing head 16 causes an error signal to appear in conductors 27 and 261' between the signal generated by radiation sensing head 16 and the signal generated by wiper 56a of potentiometer 56. The voltage polarity of the signal in conductors 27 and 261 depends on whether the temperature of web W is above or below the desired temperature. For example, a web temperature above the desired temperature may provide a positive voltage in conductor 261 while a web temperature below the desired temperature may provide a negative voltage in conductor 261. As amplifier 30 operates around a null condition, the magnitude of the error signal is of lesser importance than its I polarity.

The aforesaid error signal is applied to the remaining portions of amplifier 30 which operate electropneumatic transducer and ratio valve mechanism Yin a manner to reduce the input error signal to zero zero. Specifically, the error signal in conductors 27 and 261 is applied to control windings 92 and 94 of the magnetic preamplifier 302 which provides an amplified error signal in output winding 96. Due to the pulse amplitude modulation characteristics of preamplifier 302, the output signal in output winding 96 and conductor 114 will approximate a square wave, as shown in FIG. 4. If the height of both the positive half cycle 200 and the negative half cycle 202 are equal, the net output of the amplifier will be zero, as shown in FIG. 4a. This condition exists when no input signal is applied to control windings 92 and 94. An input signal to control windings 92 and 94 of one polarity operates magnetic preamplifier 302 to increase the height of one half cycle, for example, positive half cycle 200 and decrease the height of the negative half cycle 202, forming an unsymmetrical wave form which has a positive direct currentv component 204 proportional to the error signal. (See FIG. 4b) An input signal to control windings 92 and 94 of the opposite polarity decreases the height of positive half cycle 200 and increases the height of negative half cycle 202, forming an unsymmetrical wave form which has a negative DC component 206. (See FIG. 4c)

The output signal in conductor 114 is provided to operational amplifier 303 for further amplification so that an amplified output signal appears in conductors 146 and 148.which is related in polarity to the input error signal signal.

The output signal in conductors 146 and 148 drives transducer 150. Assume in an exemplary instance'the output signal of amplifier 30 is of a polarity to indicate that the temperature of web W is below the desired temperature, as determined by the setting of wiper 56a of potentiometer 56. The signal in conductor l46and 148 energizes solenoid coil 154 to drive plunger 156 downward into cavity. 158. This causes spacer 164 to depress valve seat 162 and valve stem 174. The depression of valve stem 174 increases the air flow through valve seat 180 from chamber 182, connected to air supply line 186, to

cavity 158, connected to air line 172.'This increases the airpressure in air line 172 and in cavity 158. The increased air pressure in cavity 158 causes diaphragm 168 to flex upwardly, raising valve seat 162 and valve stem 174 against the downward force exerted by solenoid plunger 156. The opposing forces on diaphragm 160 assume a state of balance so that the air pressure in air line 172 is a function of the energization of solenoid 152 by amplifier 30 and hence of the error signal between the desired and actual temperatures of web W.

The increased air pressure in air line 172, when applied to the desired temperature, solenoid 152 is energized to withdraw solenoid plunger 156 from cavity 158. This permits valve seat 162 to be raised off valve stem 174, by means of spring 164, and allows the air flowing from chamber 182 into cavity 158 to escape through valve seat 162 and out exhaust port 168, reducing the air pressure in cavity 158 and in airline 172. The reduced pressure in cavity 158 causes diaphragm to flex downward, restricting the flow of air through valve seat 162 so that the opposing forces on the diaphragm again balance when the air pressure in air line 172 has reached a value proportional to the magnitude ofthe output signal of amplifier30. 7' i l I The decreased air pressure in.air line 172, when applied to piston 190 of air cylinder 188, causes. a retraction of spring loaded piston 190 and piston rod 192 to actuate linkage L to reduce the gas supplied by ratio valve mechanism V to burner B and decrease the temperature in oven and the temperature of web W to bring the actual temperature of the web into coincidence with the desired temperature thereof.

In the embodiment of the temperature sensing means shown in FIG. 3, the emissivity of the web material as a factor in the control system is compensated for electrically by potentiometer 165 connected in shunt with potentiometer 56. In the operation of the sensing means of FIG. 3, the web is heated to the temperature which is desired or necessary for the processing of the web. The web temperature is then measured as accurately as possible by a pyrometer or other means such as the conduction or convection temperature sensors mentioned, supra. The temperature so ascertained is then set on electric signal means 301 by adjusting the 'dial and the wiper of potentiometer 56a, even though it may not be the actual temperature of web W. Since it is not the actual temperature of web' W as sensed by radiation head 16, an error signal will be generated between conductors 27 and 261 which will be indicated by meter 104. Potentiometer. 165 is then adjusted to alter the relative parallel current flow between potentiometer 56 and potentiometer 165 and thesignal generated at wiper 56a until meter 104 indicates a condition of null balance in amplifier 30. With this compensation :inserted in resistor bridge 32, the control system will regulate the temperature of web W to the desired temperature in a manner similar to that described above. Potentiometer 165 may be provided with a dial or other indicating means onwhich'the amount of emissivity compensation required may be read. In the event it is again desired to process the same type of web material, both potentiometer 56 and potentiometer 165 may be returned to their. former settings so as to again control the temperature of the web to the same level.

It is appreciated that other modifications and variations may be made to the control system of the present invention and it is desired to include in the appended claims all such variations and modifications as'come within the true scope and spirit of the invention.

l claim:

1. A temperature control system for regulating the temperature level of a heated moving webpassing through web heat treating apparatus to a desired temperature level, said apparatus having a controllable heating means for applying heat to the web, said web emanating radiation in accordance with its temperature and an unknown emissivity factor, said system comprising:

a sensorymeans positioned adjacent said web and exposed to the radiation emanating therefrom, said means being responsive to said radiation for providing and an electric signal corresponding in magnitude to said radiation;

control means including electric signal means, said electric signal means being connected to said sensory means for providing an electric signal of a preselected magnitude in electrical opposition to the electric signal of said sensory means, said control means also containing deviation sensing means responsive to a said electric signals for determining when a condition. of magnitude equality exists between said signals and for providing an error signal corresponding to a deviation from said condition of equality, said deviation sensing means comprising a nulltype, high gain amplifier; 7 means for altering one of said electric signals from said sensory means and said electric signal means to compensate for the emissivity of said moving web, so that said electric signals are in a condition of magnitude equality when the web is at the desired temperature level; and electromechanical means responsive 'to the error signal from said deviation sensing means and connected to said heating means for controlling said heating means and the heat applied to the web in accordance with the error signal for causing the temperature level of said web to e ual the desired temperature level. 2. e temperature control system of claim 1 wherein said means for altering one of the signals comprises means including a heatable body capable of emanating radiationin accordance with its temperature and; a temperature control system connected to a said body for raising and maintaining the temperature of said body at the desired temperature level of said web, said sensory means being positioned with respect to said body for receiving only the radiation from said body, so that in the absence of said web, said sensory means provides a signal corresponding to the desired temperature level of said web, said web passing between said body and said sensory means so that deviations in the temperature thereof with respect to the desired temperature level cause corresponding deviations in the output signal of said sensory means and the condition of said electric signals.

3. The temperature control system of claim 2 wherein said heatable body comprises a U-shaped trough partially surrounding said web and providing anexposed portion of said web and trough which may be observed by said sensory means.

4. The temperature control system of claim 3 wherein said U-shaped through includes a portion lying on one side of said web, a portion positioned partially on the other side of said web, and a portion connecting the aforesaid portions, said portion positioned partially on the other side of said web exposing said web and trough for observation by'said sensory means. i

5. The temperature control system of claim 3 wherein said temperature control means includes an adjustable electrical power supply, an electric heaterconnected to the output of said power supply and to said body for heating said body, and temperature responsive means connected to said body for sensing the temperature thereof and for altering the output of said power supply for maintaining the temperature of said body at the desired temperature of said web.

6. The temperature control system ofclaim 1 wherein said means for altering one of said electricsing signals comprises means operatively associated with. said electric signal means for altering electric signals produced by said means to bring the electric signals of said sensory means and said electric signal means into a condition of magnitude equality when said web is at the desired temperature level. v

7. The temperature control system of claim 1 wherein said sensory means is responsive to the entire spectrum of radiation emanating from said web.

8. The temperature control systemof claim 7 wherein said sensory means includes a thermopile exposed to the radiation emanating from the web for providing an electric signal corresponding to the amount of said radiation and also includes a means connected to said thermopile and responsive to the ambient temperature to which the thermopile is exposed for altering said electric signal to provide ambient temperature compensation to said sensory means.

9. The temperature control system according to claim 1 wherein said amplifier is of the pulse amplitude modulation type.

10. The temperature control systemaccording to claim 1 wherein said electromechanical means includes anelectric means responsive to said error signal, pneumatic means operable by said electric means for providing pneumatic. pressure proportional to said error signal, and pneumatically operated mechanical means connected to said pneumatic means and to said heating means for controlling said heating means in accordance with the pneumatic pressure provided by said pneumatic means. 

1. A temperature control system for regulating the temperature level of a heated moving web passing through web heat treating apparatus to a desired temperature level, said apparatus having a controllable heating means for applying heat to the web, said web emanating radiation in accordance with its temperature and an unknown emissivity factor, said system comprising: a sensory means positioned adjacent said web and exposed to the radiation emanating therefrom, said means being responsive to said radiation for providing and an electric signal corresponding in magnitude to said radiation; control means including electric signal means, said electric signal means being connected to said sensory means for providing an electric signal of a preselected magnitude in electrical opposition to the electric signal of said sensory means, said control means also containing deviation sensing means responsive to a said electric signals for determining when a condition of magnitude equality exists between said signals and for providing an error signal corresponding to a deviation from said condition of equality, said deviation sensing means comprising a null type, high gain amplifier; means for altering one of said electric signals from said sensory means and said electric signal means to compensate for the emissivity of said moving web, so that said electric signals are in a condition of magnitude equality when the web is at the desired temperature level; and electromechanical means responsive to the error signal from said deviation sensing means and connected to said heating means for controlling said heating means and the heat applied to the web in accordance with the error signal for causing the temperature level of said web to equal the desired temperature level.
 2. The temperature control system of claim 1 wherein said means for altering one of the signals comprises means including a heatable body capable of emanating radiation in accordance with its temperature and a temperature control system connected to a said body for raising and maintaining the temperature of said body at the desired temperature level of said web, said sensory means being positioned with respect to said body for receiving only the radiation from said body, so that in the absence of said web, said sensory means provides a signal corresponding to the desired temperature level of said web, said web passing between said body and said sensory means so that deviations in the temperature thereof with respect to the desired temperature level cause corresponding deviations in the output signal of said sensory means and the condition of said electric signals.
 3. The temperature control system of claim 2 wherein said heatable body comprises a U-shaped trough partially surrounding said web and providing an exposed portion of said web and trough which may be observed by said sensory means.
 4. The temperature control system of claim 3 wherein said U-shaped through includes a portion lying on one side of said web, a portion positioned partially on the other side of said web, and a portion connecting the aforesaid portions, said portion positioned partially on the other side of said web exposing said web and trough for observation by said sensory means.
 5. The temperature control system of claim 3 wherein said temperature control means includes an adjustable electrical power supply, an electric heater connected to the output of said power supply and tO said body for heating said body, and temperature responsive means connected to said body for sensing the temperature thereof and for altering the output of said power supply for maintaining the temperature of said body at the desired temperature of said web.
 6. The temperature control system of claim 1 wherein said means for altering one of said electric sing signals comprises means operatively associated with said electric signal means for altering electric signals produced by said means to bring the electric signals of said sensory means and said electric signal means into a condition of magnitude equality when said web is at the desired temperature level.
 7. The temperature control system of claim 1 wherein said sensory means is responsive to the entire spectrum of radiation emanating from said web.
 8. The temperature control system of claim 7 wherein said sensory means includes a thermopile exposed to the radiation emanating from the web for providing an electric signal corresponding to the amount of said radiation and also includes a means connected to said thermopile and responsive to the ambient temperature to which the thermopile is exposed for altering said electric signal to provide ambient temperature compensation to said sensory means.
 9. The temperature control system according to claim 1 wherein said amplifier is of the pulse amplitude modulation type.
 10. The temperature control system according to claim 1 wherein said electromechanical means includes an electric means responsive to said error signal, pneumatic means operable by said electric means for providing pneumatic pressure proportional to said error signal, and pneumatically operated mechanical means connected to said pneumatic means and to said heating means for controlling said heating means in accordance with the pneumatic pressure provided by said pneumatic means. 